In a dynamic Time Division Duplex (TDD) system, a group of subframes are fixed subframes, i.e. they are either uplink or downlink subframes in all radio frames, while others are flexible subframes, i.e. in some radio frames they can be UpLink (UL) subframes while in other radio frames same subframe can be DownLink (DL) subframes. The assignment of the UL or DL direction may be performed in a dynamic manner. A flexible subframe when used as DL or UL subframe may contain only user specific data e.g. Physical DL Shared Channel (PDSCH) and/or Physical UL Shared Channel (PUSCH) and associated control information e.g. DeModulation Reference Signal (DMRS) in DL, Sounding Reference Signal (SRS) in UL etc. This means that a flexible subframe in DL may not even contain signals like Cell-specific Reference Signal (CRS), Positioning Reference Signal (PRS) etc. The PRS may be transmitted occasionally (e.g. once with a periodicity of 160 ms, 320 ms, 6400 ms or 1280 ms). The subframe containing PRS are interchangeably called as a PRS subframe. Due to the use of flexible subframes, the transmission of all desired number of PRS subframes cannot be consistently guaranteed. This in turn will degrade the positioning performance of e.g. the Observed Time Difference Of Arrival (OTDOA) which relies on PRS based User Equipment (UE) positioning measurements used for finding UE location under e.g. emergency services. Therefore under dynamic TDD the regulatory requirements may not be met. More details on dynamic TDD and positioning and the different types of subframes will be described in greater detail in the detailed description of this application.
Also, when a PRS used for OTDOA instant coincides with a flexible subframe, it may not be guaranteed that the PRS can be sent in that subframe. Therefore some PRS subframe may be unavailable for Reference Signal Time Difference (RSTD) measurement.
In OTDOA the RSTD is used for finding UE position under emergency services. Regulators require that UE location is determined within a certain delay and the location should be accurate within a certain range e.g. ±50 m. The regulatory requirements may vary depending upon region or country. The delay depends upon the RSTD measurement time and positioning accuracy depends upon RSTD measurement accuracy. The typical RSTD measurement accuracy can be e.g. ±5 Ts, where 1 Ts=32.5 ns. The RSTD measurement requirements (e.g. RSTD measurement period, RSTD measurement accuracy etc.) in turn depends upon one or more PRS configuration parameters e.g. PRS periodicity, PRS subframes per PRS occasion, PRS muting pattern, PRS bandwidth etc. For example the RSTD measurement time increases with the increase in PRS periodicity. In existing solutions without dynamic TDD the RSTD measurement time for different PRS configuration parameters are within an acceptable limit specified by regulators.
A UE receives PRS configuration and information about flexible subframes for the same cells from a positioning node and/or a network node (e.g. serving eNode B or eNB). This is shown in FIG. 1 showing reception by a UE of PRS configuration and flexible subframe information from a positioning node and a network node respectively. The eNB may transmit the configuration via Radio Resource Control signalling (RRC) as shown in FIG. 1.
A UE expects a PRS in subframes according to the PRS configuration sent to the UE by the positioning node in the OTDOA assistance information. The two sets of information from positioning and network nodes may however be conflicting in the existing system since no solution exists to address this. For example a positioning node may indicate certain subframes (e.g. subframe #3) as the valid PRS subframe(s) to be used for positioning measurements. However due to the nature of dynamic TDD, a network node may decide to use the same subframe (e.g. subframe #3) as flexible subframe and inform the UE about this. Hence they may collide.
A flexible subframe even in DL may not contain PRS. This will have severe consequences on the positioning measurement performance. For example the UE may receive an inadequate number of PRS subframes to meet a desired accuracy. The UE may also perform measurements on signals in inappropriate subframes which don't actually contain PRS. This will results in longer measurement time i.e. delay; e.g. fewer available PRS subframes and/or degraded measurement accuracy e.g. measuring on incorrect signals. In both cases the desired positioning accuracy based on OTDOA measurements will not be fulfilled. Since OTDOA is used for emergency services a degraded accuracy is not considered acceptable by the regulatory requirements.